There are around 44 to 66 million tonnes of nitrogen fixed from atmospheric nitrogen by symbiosis of rhizobial microorganisms and legumes every year, which is nearly half of the nitrogen used in agriculture around the world.
The interaction of legume and rhizobia is highly specific and each rhizobial species has a distinct range of leguminous plant hosts for forming nodules and fixing nitrogen. Different steps of the nodulation process requires the exchange of various signals between host legumes and rhizobia.
Actinobacteria are a large group that includes different genera of Gram-positive bacteria with a high G-C content in their DNA. Actinobacteria are widely distributed in terrestrial environments and some, like the nitrogen-fixing symbionts Frankia, are known to form associations with plants through symbiotic relationships. Recent studies have also found that endophytic actinoacteria produced plant growth-promoting compounds such as indole-3-acetic acid (IAA) and siderophores.
Effects of actinobacteria on rhizobia and symbiosis with legumes have been noticed but not many studies have investigated this complex combination, although several studies have suggested antagonism occurring between actinobacteria and rhizobia.
For example, Antoun et al. (Canadian Journal of Microbiology 24: 558-562, 1978) disclosed antagonism tests between actinobacteria isolated from different soils and effective strains of rhizobia. These data demonstrated that some actinobacteria inhibit the growth of rhizobia in vitro and in planta. Specifically, thirty one percent of the 481 actinobacteria investigated inhibited two efficient rhizobia strains, Rhizobium meliloti A2 and S14.
In a further study, Damirgi and Johnson (Agronomy Journal 58: 223-224, 1966) disclosed that the number of nodules on soybeans inoculated with Rhizobium japonicum strains 122 and 123 in autoclaved soil were reduced by up to 35% and 53%, respectively, by treatment with the actinobacterium E8. They also isolated about 60 actinobacteria from one soil sample where there had been poor nodulation of clovers. However, 20 of 24 actinobacteria isolated from an experimental soybean field did not inhibit eight sensitive R. japonicum strains in in vitro tests.
Antagonism was also examined between actinobacteria and 12 strains of rhizobia from five soil samples by Patel (Plant and Soil 41: 395-402, 1974). It was reported that about 23-70% of the actinobacteria inhibited the rhizobia strains.
In light of the above, identification of actinobacterial inoculants that are compatible with rhizobial microorganisms and that can enhance the growth and development of leguminous plants would be desirable.